The electric arc furnace process is a common steel making practice in use today. In a typical electric arc furnace process, solid charge ingredients including raw scrap, limestone, burnt lime, iron ore and ferro-alloy additives are placed in a top-charge furnace unit.
A conventional furnace unit is equipped with (1) a roof lift and swing arrangement which permits the roof to swing aside when cold scrap is charged into the furnace, (2) a rocker and rail tilting type arrangement which permits the furnace to tilt forward for tapping and backward for slagging, (3) a system for additions through the furnace roof, and (4) evacuation systems for the removal of dust generated during the steel making cycle.
Electrodes are supported by electrode arms and clamps and project from overhead down through the furnace roof. An electric arc surging between the electrodes and through the furnace charge, typically comprising largely scrap metal, produces heat which melts the charge and refines the steel. The molten steel is tapped, typically at about 3000.degree. F., into a ladle and cast into blooms or poured into ingot molds.
In such a process, particulate emissions are generated during (1) charging of scrap, (2) tapping of furnaces, (3) pneumatic injection of additives, (4) oxygen blowing, and (5) meltdown/refining periods. This particulate, which is individually and collectively referred to as electric arc furnace dust (hereinafter EAFD), is typically collected either as a dry waste in baghouses or wet, as sludge.
In its emitted form, EAFD readily leaches heavy metals when wet, producing heavy metals concentrations in leachate which exceed the limits as set forth by the United States Environmental Protection Agency (EPA). In fact, EAFD is designated hazardous by the EPA and carries the designation of "KO61" as a hazardous material because of the presence of relatively high amounts of leachable heavy metals, for example, lead, chromium, cadmium, and thallium. As a hazardous material, untreated EAFD can be disposed of only in landfills designated to accommodate hazardous materials. Such landfills are more costly than landfills designated to accommodate non-hazardous materials. In addition, the transportation and handling of hazardous materials are more expensive than the transportation and handling of non-hazardous materials.
The EPA has set forth certain tests to assess whether a particular process is deemed to transform EAFD into a product which may be placed in any landfill, either hazardous or non-hazardous. These tests are described in Appendix II of 40 C.F.R. .sctn. 261, which sets forth a two-stage testing procedure for making a determination as to whether a newly-presented procedure transforms EAFD into an acceptable product. The first stage of this two-stage procedure is known as the EPA's Toxicity Characteristic Leaching Procedure (TCLP), which involves exposing the material to be tested to an acetic acid solution for sixteen hours. The second stage is known as the Multiple Extraction Procedure (MEP), which involves exposing the solids filtered from the TCLP to nine sequential baths of acids, including nitric and sulfuric acids. This test has been deemed equivalent to exposure to 1,600 hours of acid rainfall.
After completion of the MEP, the leachate is measured for fourteen metals. If the concentrations in the leachate of thirteen of these metals are less than the concentration limits specified by the EPA as its Land Disposal Restriction (LDR) limits, then the EAFD treated by the tested process may be disposed of in a landfill accommodating hazardous wastes. More desirably, if the concentrations in the leachate of all fourteen metals are less than the concentration limits specified below (i.e., the Generic Delisting Limits (GDL)), then the material is deemed non-hazardous and may be disposed of in any landfill:
TABLE 1 ______________________________________ Antimony 0.06 mg/l Arsenic 0.50 mg/l Barium 7.6 mg/l Beryllium 0.010 mg/l Cadmium 0.050 mg/l Chromium 0.33 mg/l Lead 0.15 mg/l Mercury 0.009 mg/l Nickel 1 mg/l Selenium 0.16 mg/l Silver 0.30 mg/l Thallium 0.020 mg/l Vanadium 2 mg/l Zinc 70 mg/l ______________________________________
The current state of the art for treatment and disposal of EAFD is either high temperature processing or chemical stabilization/fixation. For technical and economic reasons, the chemical stabilization/fixation treatment is growing rapidly in use and was performed on over one-third of the approximately 750,000 tons of EAFD generated in 1997 in the United States.
Several methods of chemically stabilizing EAFD have been disclosed. For example, U.S. Pat. Nos. 4,840,671 and 4,911,757, entitled PROCESS FOR CHEMICAL STABILIZATION OF HEAVY METAL BEARING DUSTS AND SLUDGES and issued to Lynn et al., disclose methods and mixtures for stabilizing EAFD and similar dusts. The methods disclosed in these two references include mixing EAFD with fly ash, lime, and water, among other ingredients. This method relies primarily on the pozzolanic characteristics of fly ash to physically entrap the hazardous constituents of EAFD within a cementitiously hardened product.
U.S. Pat. No. 5,245,122, entitled METHOD AND MIXTURE FOR TREATING ELECTRIC ARC FURNACE DUST, discloses a method for chemically stabilizing a hazardous waste composition containing EAFD by utilizing the pozzolanic characteristics of EAFD. This method involves forming a mixture of EAFD with water and lime and, optionally, ferrous sulfate. The freshly blended product has acceptable leachate concentrations. The method disclosed in the '122 patent minimizes the concentration of certain heavy metals in the leachate from the freshly blended product. U.S. Pat. No. 5,569,152, entitled BUFFERING OF CEMENTITIOUS HAZARDOUS WASTE COMPOSITIONS CONTAINING ELECTRIC ARC FURNACE DUST, specifies the use of dolomitic lime as a buffering agent for a cementitious hazardous waste composition including EAFD.
The composition of EAFD is dependent on the type of metal used as scrap added to the electric arc furnace. In recent years, there has been a trend of using scrap metals having an increased thallium content as additives to electric arc furnaces. Accordingly, the EAFD derived from electric arc furnaces has included more and more thallium. There is a need for a stabilization process of EAFD which immobilizes, or limits the leachability of, thallium.